1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for manufacturing the same. More particularly, the present invention provides a new liquid crystal display device for realizing gradation display and a method for manufacturing the same.
2. Related Art
In a liquid crystal display device, gradation display plays an important part in display performance. Various methods for gradation display are provided. However, it is not easy to achieve gradation display in a liquid crystal element which is fundamentally a two-valued device such as a ferroelectric liquid crystal element.
A ferroelectric liquid crystal display device generally utilizes a liquid crystal phase of chiral smectic C phase for display. This liquid crystal phase possesses, in a bulk state, a molecular sequence with a helical structure. However, when injected into a liquid crystal cell whose distance between the substrates is shorter than the helical pitch, helices in the liquid crystal phase get loose, assuming a layered structure of a plurality of layers 121 laminated in parallel in which the liquid crystal molecules 120 arrange themselves tilted with respect to the layers 121 as shown in FIGS. 9 (a) and (b).
When electric field is applied to the liquid crystal phase having the above structure, bistable two states appear, namely the state shown in FIG. 9 (a) and the state shown in FIG. 9 (b). This is because ferroelectric liquid crystals have spontaneous polarization (Ps) in a direction perpendicular to the printed surface in FIGS. 9 (a) and (b) and, when electric field (E) is applied in the direction, liquid crystal molecules 120 rearrange themselves to array spontaneous polarization in the direction of the electric field.
By interposing liquid crystal cells containing such liquid crystals between a pair of polarizing plates (an optical polarizer and an optical detector), selective switching of display between the bright state of FIG. 9 (a) and the dark state of FIG. 9 (b) will be realized (N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 36,899 (1980)).
The above switching between the state of FIG. 9 (a) and the state of FIG. 9 (b) is performed by a close interaction of the electric field and the spontaneous polarization, so that a high speed response in the order of micro seconds will be realized by switching the direction of applied electric field. Besides, ferroelectric liquid crystals have what is known as a memory property, a property to retain, after the electric field is turned off, the state before the electric field is turned off. Therefore, by utilizing high speed response and memory property, display contents can be written in with a high speed per each scanning line, whereby a simple-matrix type display device having large display capacity can be obtained.
FIG. 12 (a) shows a fundamental structure of a liquid crystal display device using a ferroelectric liquid crystal. On the two glass substrates 201 of this liquid crystal display device are formed electrode films 202 of ITO (Indium Tin Oxide), on which are formed insulating films 203 and orientation films 204. Orientation films 204 are generally made of high polymer film such as polyimide and a rubbing treatment is applied to their surface. Those two substrates 201 are bonded together with the cell thickness of about 1.5 .mu.m, and between the two substrates 201 is injected a liquid crystal 205 whose surroundings are sealed with a sealing material 206. On front and rear sides of the liquid crystal cell, there is provided a pair of polarizing plates, for example, an optical detector 207 on one side and an optical polarizer 211 on the other side. To each of the electrode films 202 is connected a driving circuit (not shown).
Ferroelectric liquid crystal display devices with such a structure are no different from the conventional simple-matrix type liquid crystal display device shown in FIG. 12 (b) except that the cell thickness is as thin as about 1.5 .mu.m and that the liquid crystal 205 is a ferroelectric liquid crystal. In FIG. 12 (b), the same number represents the corresponding part in FIG. 12 (a) and 205a in the Figure represents a liquid crystal which is not ferroelectric.
Various methods are proposed to be employed as a gradation display method using the ferroelectric liquid crystals. For example, Japanese Patent Laid-Open (Kokai) Publication No. SHO 62(1987)-145216 discloses the art of obtaining gradation display by continuously varying the cell thickness in a pixel. According to the art, difference in cell thickness induces the electric field intensity to vary and gradation display is obtained by controlling the area ratio of the switching region to the non-switching region.
Meanwhile, Fujikake et al. reported that gradation display can be obtained by injecting into ferroelectric liquid crystal cells a mixture solution of ferroelectric liquid crystal and photo-curable prepolymer and then applying light into the cells to photo-polymerize the prepolymer (Fujikake et al., the 41st Joint Lecture Meeting related to Applied Physics, preliminary lecture drafts No. 3, 1120 (1994)). According to the method, the composite of ferroelectric liquid crystal and resin forms a domain structure by photo-polymerization of the prepolymer, and gradation display is obtained by controlling the area ratio of the switching regions utilizing the difference of threshold property in each domain.
However, the above methods generate problems such as described below.
The method of Japanese Patent Laid-Open (Kokai) Publication No. SHO 62(1987)-145216 has a problem that, because the difference in cell thickness causes different molecular orientation, it is hard to obtain, for example, good display in black with sufficiently low amount of transmitted light. Particularly, if a material with negative dielectric anisotropy is used as a ferroelectric liquid crystal material and the driving operation is performed utilizing the specific .sup..tau.-V min characteristics of the material, memory angle is affected by the electric field intensity when a bias voltage is applied, so that the difference in cell thickness induces different memory angle (different direction of the major molecular axis) in each region, failing to provide display in complete black. Another problem is that a level difference causes irregular orientation, leading to leakage of light therefrom. Moreover, there is a drawback such that continuously changing the cell thickness involves more manufacturing processes, leading to increased costs.
On the other hand, according to the method of Fujikake et al., it is not possible to exactly control the threshold property distribution of the ferroelectric liquid crystal in each pixel, so that it is not easy to provide continuous gradation display characteristics equally in every pixel. Besides, it is necessary to provide domain size sufficiently smaller than the pixel size, which is not always easy to achieve.